1. Field of the Invention
This invention relates to a process for preparing a foamed body having chemically embossed patterns. More particularly, the present invention relates to chemicaly embossed patterns derived from sheets containing natural or synthetic rubber and a vinyl chloride resin.
2. Description of the Prior Art
In general, there are two methods for forming embossed patterns. One is the so-called "Valley" printing method in which embossing is effected simultaneously with printing, and the other is the so-called chemical-embossing method. Many proposals have been made particularly with regard to the latter method, as described in U.S. Pat. No. 3,293,094 and Japanese Patent Publication No. 43-28636, for example, which disclose methods for producing embossed patterns in which the surface of a sheet of an unfoamed resin composition containing a blowing agent, is printed with an ink which contains a decomposition inhibitor. The latter ingredient inhibits the decomposition of the blowing agent, so that the decomposition process at the printed surfaces is retarded whereby embossed patterns are produced. Japanese Patent Publication No. 43-15713 discloses a method for producing printed chemically embossed patterns in which a sheet of an unfoamed resin composition containing a blowing agent is printed with an ink which contains at least one substance which functions to lower the decomposition temperature of the blowing agent, such as a so-called "kicker" or the like. As a result, the foaming process at the printed surfaces is accelerated resulting in an embossing effect. In addition, U.S. Pat. No. 3,804,933, and Japanese Laid-Open Patent Applications No. 48-84158 and 49-105853 disclose methods for producing printed chemically embossed patterns which utilize sheets of a monomer having unsaturated bonds or of an unfoamed resin composition containing a reactive plasticizer and a blowing agent. The surfaces of these sheets are printed with an ink containing peroxides or amines whereby the printed portions are inhibited from foaming, resulting in production of embossed patterns. However, the methods disclosed in the first three mentioned references require very precise control of the heating conditions used to decompose the blowing agents. Moreover, the methods of the last three mentioned references suffer from the disadvantage that evenness of the sheet is not assured because the surface finish depends greatly upon the degree of bridging in the composition. As a result, it is difficult to obtain a uniform sheet. Furthermore, since the thickness of the sheet is not constant or uniform, the printed portions often are discolored, e.g., are whitened, and become cracked.
Yet another technique of chemically embossing a plastic material is disclosed by Crowley, in U.S. Pat. No. 3,453,171, in which a polymeric resin such as a vinyl resin containing a synthetic or natural rubber component and also containing a blowing agent is coated in a pattern with an ink containing hardening or cross-linking agents for the polymeric and rubber components of the composition. Thus, materials such as peroxides, azo compounds, and the like are employed to cure the polymeric component while materials such as sulfur and zinc oxide are employed to vulcanize the rubber component of the plastic composition. Once the plastic composition is coated, it is embossed by heating the composition. In the regions coated by the ink containing hardening agents, foaming of the plastic composition is retarded relative to the uncoated areas thus effecting chemical embossing.
In addition to the above techniques disclosed by Crowley, it is also known when a natural or synthetic rubber containing a blowing agent having a decomposition temperature higher than the curing and cross-linking temperature is sufficiently cured and cross-linked under the described conditions and then is subjected to a temperature higher than the decomposition temperature of the blowing agent, foaming while controlled, hardly occurs. It is also known that a natural or synthetic rubber can be cross-linked or cured by sulfur, zinc oxide or the like and that a reactive monomer or a polymerizable monomer containing unsaturated groups can be cross-linked with organic peroxides and the like. However, despite the fact that a number of techniques are known by which chemical embossing can be effected, none of them are completely satisfactory because the preparation of an embossed pattern involves the control of a number of factors. Accordingly, a need continues to exist for an improved chemical embossing technique.